Semiconductor layer including compositional inhomogeneities

ABSTRACT

A device comprising a semiconductor layer including a plurality of compositional inhomogeneous regions is provided. The difference between an average band gap for the plurality of compositional inhomogeneous regions and an average band gap for a remaining portion of the semiconductor layer can be at least thermal energy. Additionally, a characteristic size of the plurality of compositional inhomogeneous regions can be smaller than an inverse of a dislocation density for the semiconductor layer.

REFERENCE TO RELATED APPLICATIONS

The current application is a continuation-in-part application of U.S.application Ser. No. 14/285,738, titled “Semiconductor Layer IncludingCompositional Inhomogeneities,” which was filed on 23 May 2014, andwhich claims the benefit of U.S. Provisional Application No. 61/826,788,titled “Semiconductor Layer with Compositional Inhomogeneities,” whichwas filed on 23 May 2013, and U.S. Provisional Application No.61/943,162, titled “Group III Nitride Semiconductor Composition andUltraviolet Optoelectronic Device Containing the Same,” which was filedon 21 Feb. 2014, all of which are hereby incorporated by reference.

TECHNICAL FIELD

The disclosure relates generally to emitting devices, and moreparticularly, to an emitting device including a semiconductor layer withcompositional inhomogeneous regions.

BACKGROUND ART

Compositional band fluctuations were first considered forindium-gallium-nitride (InGaN) systems. It was found that the materialproperties of InGaN alloys change as the amount of indium in the alloyis increased. With the proper growth conditions, however, it wasdiscovered that a material could be grown in which the indium did notincorporate uniformly throughout the InGaN layer (i.e., the material hadareas of high and low concentrations of indium spread throughout). Thesecompositional fluctuations, also known as localized inhomogeneities,result in carrier localization and lead to an enhancement in theradiative efficiency despite the high dislocation density. The discoveryof the effects of the localized inhomogeneities enabled the developmentof commercially successful blue InGaN-based LEDs and laser diodes (LDs).It has been reported that the intense red-shifted photoluminescence (PL)peaks observed in InGaN alloys at room temperature result from therecombination of excitons localized at potential minima originating fromlarge compositional fluctuations.

Similar localization effects were observed foraluminum-indium-gallium-nitride (AlInGaN) and aluminum-gallium-nitride(AlGaN) systems. The use of aluminum gallium nitride (Al_(x)Ga_(1-x)N),as opposed to InAlGaN, is currently preferred as the base material formanufacturing ultraviolet (UV) light emitting diode (LED) devices forultraviolet semiconductor optical sources operating at wavelengthsbetween 260 to 360 nanometers (nm) due to its tunable band gap from 3.4eV to 6.2 eV.

One approach discloses a semiconductor structure containingcompositional fluctuations as well as a method for depositing groupIII-nitride films called molecular beam epitaxy (MBE). The structurecomprises self-assembled nanometer-scale localized compositionallyinhomogeneous regions. Within these regions, the luminescence occurs dueto radiative recombination of carriers in the self-assemblednanometer-scale localized compositionally inhomogeneous regions havingband-gap energies less than surrounding material. Further, anotherapproach discloses self-assembled nanometer-scale localizedcompositionally inhomogeneous regions that include a fine scale facettedsurface morphology or pits with diameters of about ten to one hundrednanometers. The approach also discloses the semiconductor devicecomprising of such semiconductor structures.

Group-III nitride based semiconductors are materials of choice forultraviolet light emitting diodes, photomultipliers and photodiodes.Currently, wall plug operating efficiencies of deep ultraviolet lightemitting devices reach only a few percent and a large effort is devotedto improving their efficiency.

Similar to InGaN-based semiconductor devices, carrier localization playsan important role in light emission from devices based on AlGaNsemiconductor layers. Even though these materials are typically grownwith a large number of threading dislocations and point defects,emission efficiency is higher than anticipated and radiative lifetimesobtained from photoluminescence studies are smaller than predicted bytheory. This effect can be attributed to the carriers being isolatedfrom nonradiative recombination centers due to localization in sitescontaining a smaller band gap than the surrounding semiconductormaterial.

FIG. 1 shows a schematic of compositional fluctuation according to theprior art. During the initial growth stage, adjacent small islands, fromwhich the growth starts, coalesce into larger grains. As the islandsenlarge, Ga adatoms, having a larger lateral mobility than Al adatoms,reach the island boundaries more rapidly. As a result, the Gaconcentration in the coalescence regions is higher than in the center ofthe islands. The composition pattern, which is formed during thecoalescence, is maintained as the growth proceeds vertically. As aresult of the coalescence, the domain boundaries usually containextended defects that form to accommodate the relative difference incrystal orientation among the islands. Even in layers with smoothsurfaces containing elongated layer steps, screw/mixed dislocationsoccur due to the local compositional inhomogeneities.

SUMMARY OF THE INVENTION

In light of the above, the inventors recognize that compositionalinhomogeneous regions in a semiconductor layer of a device can allow forincreasing radiative recombination of carriers and decreasingnonradiative recombination time by preventing electrons from reachingthreading dislocation cores.

Aspects of the invention provide a device comprising a semiconductorlayer including a plurality of compositional inhomogeneous regions,which can be configured to, for example, improve internal quantumefficiency (IQE) and the overall reliability of the device. A differencebetween an average band gap for the plurality of compositionalinhomogeneous regions and an average band gap for a remaining portion ofthe semiconductor layer can be at least thermal energy. Additionally, acharacteristic size of the plurality of compositional inhomogeneousregions can be smaller than an inverse of a dislocation density for thesemiconductor layer.

A first aspect of the invention provides a device comprising: asemiconductor layer comprising a plurality of compositionalinhomogeneous regions, wherein a difference between an average band gapfor the plurality of compositional inhomogeneous regions and an averageband gap for a remaining portion of the semiconductor layer is at leastthermal energy, and wherein a characteristic size of the plurality ofcompositional inhomogeneous regions is smaller than an inverse of adislocation density for the semiconductor layer.

A second aspect of the invention provides a device comprising: asemiconductor structure including an active region, wherein the activeregion comprises a multiple quantum well structure including: aplurality of barriers alternating with a plurality of quantum wells,wherein at least one of: a barrier in the plurality of barriers or aquantum well in the plurality of quantum wells includes a plurality ofcompositional inhomogeneous regions, wherein a difference between anaverage band gap for the plurality of compositional inhomogeneousregions and an average band gap for a remaining portion of thesemiconductor layer is at least thermal energy, and wherein acharacteristic size of the plurality of compositional inhomogeneousregions is smaller than an inverse of a dislocation density for thesemiconductor layer.

A third aspect of the invention provides a method comprising: forming anactive region of a semiconductor structure, wherein the active regioncomprises a light emitting heterostructure, the forming including:forming a plurality of barriers alternating with a plurality of quantumwells, wherein forming at least one of: a barrier in the plurality ofbarriers or a quantum well in the plurality of quantum wells includesforming a plurality of compositional inhomogeneous regions, wherein anaverage band gap for the plurality of compositional inhomogeneousregions exceeds a thermal energy of a remaining portion of thesemiconductor layer and a characteristic size for each compositionalinhomogeneous region is smaller than an inverse of a dislocationdensity.

The illustrative aspects of the invention are designed to solve one ormore of the problems herein described and/or one or more other problemsnot discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the disclosure will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various aspects of the invention.

FIG. 1 shows a schematic of compositional fluctuation according to theprior art.

FIG. 2 shows a schematic structure of an illustrative emitting deviceaccording to an embodiment.

FIG. 3A shows a hybrid structure/band diagram corresponding to a portionof an active region of an illustrative emitting device, while FIG. 3Bshows a plane of the quantum well within the active region of the deviceaccording to an embodiment.

FIG. 4A shows an illustrative plane within a multiple quantum wellstructure for a device according to an embodiment, while FIG. 4B showsan illustrative band gap map for the plane as a function of the y-axisaccording to an embodiment.

FIG. 5 shows a hybrid structure/band diagram of an illustrative multiplequantum well structure according to an embodiment.

FIG. 6A shows a hybrid structure/band diagram of an illustrativemultiple quantum well structure according to an embodiment, while FIG.6B shows a plane at an interface between a quantum well and a barrierwithin the illustrative multiple quantum well structure according to anembodiment.

FIG. 7A shows a hybrid structure/band diagram of an illustrativemultiple quantum well structure according to an embodiment, while FIG.7B shows a plane of a tilted quantum well within the illustrativemultiple quantum well structure according to an embodiment.

FIG. 8A shows additional details of a hybrid structure/band diagram ofan illustrative plane within a multiple quantum well structure of adevice according to an embodiment, while FIG. 8B shows an illustrativeband gap map for the plane as a function of the y-axis according to anembodiment.

FIGS. 9A and 9B show band diagrams of portions of illustrative multiplequantum well structures according to embodiments.

FIGS. 10A and 10B show hybrid structure/band diagrams of portions of anillustrative multiple quantum well structure according to an embodiment.

FIG. 11 shows a band diagram of an illustrative quantum well accordingto embodiment.

FIGS. 12A-12C show illustrative heterostructures according toembodiments.

FIGS. 13A and 13B show band diagrams of illustrative quantum wellsaccording to embodiments.

FIGS. 14A-14C show illustrative topographical images corresponding tosample AlGaN layers with increasing Al molar fractions.

FIGS. 15A-15C show illustrative maps corresponding to sample AlGaNlayers with increasing Al molar fractions.

FIGS. 16A-16C show illustrative maps corresponding to sample AlGaNlayers with increasing Al molar fractions.

FIG. 17 shows a portion of an illustrative layer according to anembodiment.

FIGS. 18A and 18B show illustrative strain modulation for reducingthreading dislocations for a device according to an embodiment.

FIGS. 19A and 19B show illustrative bright field optical microscopeimages of layers according to an embodiment.

FIG. 20 shows a graph showing the reduction of the full width at halfmaximum (FWHM) as a function of increasing the AlN layer thicknessaccording to an embodiment.

FIGS. 21A and 21B show illustrative patterning for compressive andtensile layers in a device according to an embodiment.

FIG. 22 shows an illustrative contact to a layer including compositionalinhomogeneities according to an embodiment.

FIGS. 23A and 23B show an illustrative contact to a layer includingcompositional inhomogeneities and compositional variations according toan embodiment.

FIG. 24 shows an illustrative contact including a plurality of metallicprotrusions to a semiconductor layer including a plurality ofcompositional inhomogeneous regions according to an embodiment.

FIGS. 25A-25C show illustrative etched surfaces of a semiconductor layeraccording to embodiments.

FIGS. 26A and 26B show illustrative etched surfaces of a semiconductorlayer with non-uniform etching according to embodiments.

FIG. 27 shows an illustrative flow diagram for fabricating a circuitaccording to an embodiment.

It is noted that the drawings may not be to scale. The drawings areintended to depict only typical aspects of the invention, and thereforeshould not be considered as limiting the scope of the invention. In thedrawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, aspects of the invention provide a device comprisinga semiconductor layer including a plurality of compositionalinhomogeneous regions, which can be configured to, for example, improveinternal quantum efficiency (IQE) and the overall reliability of thedevice. A difference between an average band gap (e.g., an energydifference between a top of the valence band and a bottom of theconduction band in the semiconductor) for the plurality of compositionalinhomogeneous regions and an average band gap for a remaining portion ofthe semiconductor layer can be at least thermal energy. Additionally, acharacteristic size of the plurality of compositional inhomogeneousregions can be smaller than an inverse of a dislocation density for thesemiconductor layer. As used herein, a depth of a compositionalinhomogeneous region is defined as a difference between the conductiveband energy level at the location of the compositional inhomogeneousregion and the average conductive band energy level, which is theaverage between the hills and valleys of the energy landscape of thesemiconductor layer. As also used herein, a lateral area of thecompositional inhomogeneous regions comprises the physical areacorresponding to the location of the compositional inhomogeneous region.As used herein, unless otherwise noted, the term “set” means one or more(i.e., at least one) and the phrase “any solution” means any now knownor later developed solution.

Turning to the drawings, FIG. 2 shows a schematic structure of anillustrative emitting device 10 according to an embodiment. In a moreparticular embodiment, the emitting device 10 is configured to operateas a light emitting diode (LED), such as a conventional or superluminescent LED. Alternatively, the emitting device 10 can be configuredto operate as a laser diode (LD) or a photo-detector. When operated asan emitting device 10, application of a bias comparable to the band gapresults in the emission of electromagnetic radiation from an activeregion 18 of the emitting device 10. The electromagnetic radiationemitted by the emitting device 10 can comprise a peak wavelength withinany range of wavelengths, including visible light, ultravioletradiation, deep ultraviolet radiation, infrared light, and/or the like.In an embodiment, the device is configured to emit radiation having adominant wavelength within the ultraviolet range of wavelengths. In amore specific embodiment, the dominant wavelength is within a range ofwavelengths between approximately 210 and approximately 350 nanometers.

The emitting device 10 includes a heterostructure comprising a substrate12, a buffer layer 14 adjacent to the substrate 12, an n-type claddinglayer 16 (e.g., an electron supply layer) adjacent to the buffer layer14, and an active region 18 having an n-type side 19A adjacent to then-type cladding layer 16. Furthermore, the heterostructure of theemitting device 10 includes a p-type layer 20 (e.g., an electronblocking layer) adjacent to a p-type side 19B of the active region 18and a p-type cladding layer 22 (e.g., a hole supply layer) adjacent tothe p-type layer 20.

In a more particular illustrative embodiment, the emitting device 10 isa group III-V materials based device, in which some or all of thevarious layers are formed of elements selected from the group III-Vmaterials system. In a still more particular illustrative embodiment,the various layers of the emitting device 10 are formed of group IIInitride based materials. Group III nitride materials comprise one ormore group III elements (e.g., boron (B), aluminum (Al), gallium (Ga),and indium (In)) and nitrogen (N), such that B_(W)Al_(X)Ga_(Y)In_(Z)N,where 0≦W, X, Y, Z≦1, and W+X+Y+Z=1. Illustrative group III nitridematerials include binary, ternary and quaternary alloys such as, AlN,GaN, InN, BN, AlGaN, AlInN, AIBN, AlGaInN, AlGaBN, AlInBN, and AlGaInBNwith any molar fraction of group III elements.

An illustrative embodiment of a group III nitride based emitting device10 includes an active region 18 (e.g., a series of alternating quantumwells and barriers) composed of In_(y)Al_(x)Ga_(1-x-y)N,Ga_(z)In_(y)Al_(x)B_(1-x-y-z)N, an Al_(x)Ga_(1-x)N semiconductor alloy,or the like. Similarly, both the n-type cladding layer 16 and the p-typelayer 20 can be composed of an In_(y)Al_(x)Ga_(1-x-y)N alloy, aGa_(z)In_(y)Al_(x)B_(1-x-y-z)N alloy, or the like. The molar fractionsgiven by x, y, and z can vary between the various layers 16, 18, and 20.The substrate 12 can be sapphire, silicon carbide (SiC), silicon (Si),GaN, AlGaN, AlON, LiGaO₂, or another suitable material, and the bufferlayer 14 can be composed of AlN, an AlGaN/AlN superlattice, and/or thelike.

As shown with respect to the emitting device 10, a p-type metal 24 canbe attached to the p-type cladding layer 22 and a p-type contact 26 canbe attached to the p-type metal 24. Similarly, an n-type metal 28 can beattached to the n-type cladding layer 16 and an n-type contact 30 can beattached to the n-type metal 28. The p-type metal 24 and the n-typemetal 28 can form ohmic contacts to the corresponding layers 22, 16,respectively. In an embodiment, the p-type metal 24 and the n-type metal28 each comprise several conductive and reflective metal layers, whilethe n-type contact 30 and the p-type contact 26 each comprise highlyconductive metal. In an embodiment, the p-type cladding layer 22 and/orthe p-type contact 26 can be transparent (e.g., semi-transparent ortransparent) to the electromagnetic radiation generated by the activeregion 18. For example, the p-type cladding layer 22 and/or the p-typecontact 26 can comprise a short period superlattice lattice structure,such as a transparent magnesium (Mg)-doped AlGaN/AlGaN short periodsuperlattice structure (SPSL). Furthermore, the p-type contact 26 and/orthe n-type contact 30 can be reflective of the electromagnetic radiationgenerated by the active region 18. In another embodiment, the n-typecladding layer 16 and/or the n-type contact 30 can be formed of a shortperiod superlattice, such as an AlGaN SPSL, which is transparent to theelectromagnetic radiation generated by the active region 18.

As used herein, a layer is transparent to radiation of a particularwavelength when the layer allows a significant amount of the radiationradiated at a normal incidence to an interface of the layer to passthere through. For example, a layer can be configured to be transparentto a range of radiation wavelengths corresponding to a peak emissionwavelength for the light (such as ultraviolet light or deep ultravioletlight) emitted by the active region 18 (e.g., peak emissionwavelength+/−five nanometers). As used herein, a layer is transparent toradiation if it allows more than approximately five percent of theradiation to pass there through. In a more particular embodiment, atransparent layer is configured to allow more than approximately tenpercent of the radiation to pass there through. Similarly, a layer isreflective when the layer reflects at least a portion of the relevantelectromagnetic radiation (e.g., light having wavelengths close to thepeak emission of the active region). In an embodiment, a reflectivelayer is configured to reflect at least approximately five percent ofthe radiation. In a more particular embodiment, a reflective layer has areflectivity of at least thirty percent for radiation of the particularwavelength radiated normally to the surface of the layer. In a moreparticular embodiment, a highly reflective layer has a reflectivity ofat least seventy percent for radiation of the particular wavelengthradiated normally to the surface of the layer.

As further shown with respect to the emitting device 10, the device 10can be mounted to a submount 36 via the contacts 26, 30. In this case,the substrate 12 is located on the top of the emitting device 10. Tothis extent, the p-type contact 26 and the n-type contact 30 can both beattached to a submount 36 via contact pads 32, 34, respectively. Thesubmount 36 can be formed of aluminum nitride (AlN), silicon carbide(SiC), and/or the like.

Any of the various layers of the emitting device 10 can comprise asubstantially uniform composition or a graded composition. For example,a layer can comprise a graded composition at a heterointerface withanother layer. In an embodiment, the p-type layer 20 comprises a p-typeblocking layer having a graded composition. The graded composition(s)can be included to, for example, reduce stress, improve carrierinjection, and/or the like. Similarly, a layer can comprise asuperlattice including a plurality of periods, which can be configuredto reduce stress, and/or the like. In this case, the composition and/orwidth of each period can vary periodically or aperiodically from periodto period.

It is understood that the layer configuration of the emitting device 10described herein is only illustrative. To this extent, an emittingdevice/heterostructure can include an alternative layer configuration,one or more additional layers, and/or the like. As a result, while thevarious layers are shown immediately adjacent to one another (e.g.,contacting one another), it is understood that one or more intermediatelayers can be present in an emitting device/heterostructure. Forexample, an illustrative emitting device/heterostructure can include anundoped layer between the active region 18 and one or both of the p-typecladding layer 22 and the electron supply layer 16.

Furthermore, an emitting device/heterostructure can include aDistributive Bragg Reflector (DBR) structure, which can be configured toreflect light of particular wavelength(s), such as those emitted by theactive region 18, thereby enhancing the output power of thedevice/heterostructure. For example, the DBR structure can be locatedbetween the p-type cladding layer 22 and the active region 18.Similarly, a device/heterostructure can include a p-type layer locatedbetween the p-type cladding layer 22 and the active region 18. The DBRstructure and/or the p-type layer can comprise any composition based ona desired wavelength of the light generated by thedevice/heterostructure. In one embodiment, the DBR structure comprises aMg, Mn, Be, or Mg+Si-doped p-type composition. The p-type layer cancomprise a p-type AlGaN, AlInGaN, and/or the like. It is understood thata device/heterostructure can include both the DBR structure and thep-type layer (which can be located between the DBR structure and thep-type cladding layer 22) or can include only one of the DBR structureor the p-type layer. In an embodiment, the p-type layer can be includedin the device/heterostructure in place of an electron blocking layer. Inanother embodiment, the p-type layer can be included between the p-typecladding layer 22 and the electron blocking layer.

Regardless, as described herein, one or more of the semiconductor layersof the device 10 can comprise nano-scale and/or micron-scale localizedcompositional and/or doping inhomogeneous regions along the lateraldimensions of the device die. Inclusion of the inhomogeneous regions inone or more of the semiconductor layers of the device 10 can result inan improvement in the efficiency of the device 10. The inhomogeneousregions can be included in any layer of the semiconductor device 10. Tothis extent, the inhomogeneous regions can be included in a superlatticeregion, a nucleation region, a buffer layer, a cladding layer, an activeregion, and/or the like, of the device 10. In an embodiment, theinhomogeneities are incorporated into one or more injection layers, suchas the n-type cladding layer 16, the p-type layer 20, the p-typecladding layer 22, the n-type contact 30, the p-type contact 26, and/orthe like.

Additional aspects are shown and described in conjunction with a quantumwell, such as a quantum well included in the active region 18 of thedevice 10, including inhomogeneous regions as an illustrativeembodiment. Turning to FIGS. 3A and 3B, a hybrid structure/band diagramcorresponding to a portion of the active region 18 (e.g., a multiplequantum well structure including a plurality of quantum wellsalternating with a plurality of barriers) according to an embodiment isshown. In FIG. 3A, the multiple quantum well structure of the activeregion 18 is shown including only one quantum well 38 between twobarriers 40. However, it is understood that a single quantum well 38 isshown for clarity, and the quantum well structure of the active region18 can include any number of quantum wells alternating with any numberof barriers.

FIG. 3B shows a plane 38A within the quantum well 38 according to anembodiment. The plane 38A includes a plurality of compositionalinhomogeneous regions 42 and a plurality of threading dislocations 44.As shown, the compositional inhomogeneous regions 42 can be in the plane38A of the quantum well 38. In an embodiment, the in-plane dimensions ofthe inhomogeneous regions 42 are significantly larger than a thicknessof the quantum well 38 (e.g., approximately a few nanometers). However,it is understood that the compositional inhomogeneous regions 42 alsocan be across the thickness of a quantum well 38 (FIG. 5) and/or at aninterface between a quantum well 38 and a barrier 40 (FIGS. 6A and 6B).The plurality of compositional inhomogeneous regions 42 can formlocalized variations in the band gap of the quantum well 38.

FIG. 4B shows an illustrative band gap map as a function of the y-axisfor a location (x, z) on a plane 38A of a quantum well shown in FIG. 4A.As shown in FIG. 4A, the quantum well plane 38A includes a plurality ofcompositional inhomogeneous regions 42 and a plurality of threadingdislocations 44. As seen in FIG. 4B, the band gap for each of theplurality of compositional inhomogeneous regions 42 is less than theband gap of the remaining portion of the quantum well. Furthermore, theplurality of compositional inhomogeneous regions 42 are located betweenthe threading dislocations 44.

Inclusion of the plurality of compositional inhomogeneous regions 42 ina quantum well can enhance radiative recombination, which can improveIQE, delay non-radiative recombination, and/or the like. The pluralityof compositional inhomogeneous regions 42 can be located away from thethreading dislocations 44 and their corresponding concentration areas,so that a diffusion length of an electron before capture at a localizedcompositional inhomogeneous region 42 is smaller than a characteristicdistance to the threading dislocations. The delay in non-radiativerecombination can be achieved by preventing the electrons from reachingthe cores of the threading dislocations 44.

In an embodiment, an average band gap for the plurality of compositionalinhomogeneous regions 42 is less than an average band gap of theremaining portion of the quantum well 38 by at least half of a thermalvoltage multiplied by a carrier charge. In a further embodiment, adifference between an average band gap for the plurality ofcompositional inhomogeneous regions 42 and an average band gap for aremaining portion of the semiconductor layer (e.g., quantum well 38) isat least thermal energy, e.g., at least 26 meV at room temperature.

In an embodiment, e.g., to increase the IQE of a device, thecharacteristic size (e.g., the square root of the average lateral area)of the plurality of compositional inhomogeneous regions 42 is smallerthan an inverse of a threading dislocation density for the quantum well38. The characteristic size of the compositional inhomogeneous regions42 can be calculated, for example, as 1/N_(dis) ^(0.5), where N_(dis) isthe dislocation density per unit area. In an embodiment, the dislocationdensity per unit area is on the order of 10⁸ cm⁻² for samples grownusing a metalorganic chemical vapor deposition solution. Additionally, acharacteristic distance between threading dislocations 44 can be greaterthan a smallest size for a compositional inhomogeneous region 42.Furthermore, a lateral area for the plurality of compositionalinhomogeneous regions 42 can be smaller than a square of thecharacteristic distance between threading dislocations 44. Acharacteristic distance, d, between dislocations, which can be an upperbound of the characteristic size of the plurality of compositionalinhomogeneous regions 42, can be characterized by:

$\begin{matrix}{d = \frac{1}{\sqrt{N} - 1}} & (1)\end{matrix}$where N is the dislocation density. For example, if N=10⁹ dislocationsper cm², the characteristic distance, d, between the dislocations is

$d = {{\frac{1}{\sqrt{N} - 1} \approx {3*10^{- 5}\mspace{11mu}{cm}}} = {300\mspace{14mu}{{nm}.}}}$Therefore, the lateral area of the compositional inhomogeneous regions42 can be configured to be smaller than 90,000 nm². The lateral area ofthe regions 42 can be adjusted using any solution, e.g., by adjustingone or more conditions during epitaxial growth of the semiconductor(e.g., the quantum well 38).

The average distance between compositional inhomogeneous regions 42 canbe on the order of or less than an ambipolar diffusion length L. Theambipolar diffusion length L is characterized by:L=(D _(a)τ)^(0.5)  (2)wherein D_(a) is the am bipolar diffusion coefficient and τ is theoverall recombination time. For example, in an embodiment, D_(a) forAlGaN is approximately 10 cm²/s, so the diffusion length L can be on theorder of 1 micron for τ at approximately one nanosecond.

The internal quantum efficiency (IQE) of a device also can depend on thedensity, average lateral size, as well as the depth of the compositionalinhomogeneous regions. Furthermore, the IQE can depend on the Augerrecombination at high injection levels. For example, considercompositional inhomogeneous regions of a small size having a certaindensity throughout the semiconductor layer, which is small enough forthe compositional inhomogeneous regions to have substantially nooverlap. In this case, an expected concentration of carriers at suchlocalization centers will be higher than the average concentration,thereby leading to smaller radiation recombination times at suchregions. This may increase the IQE under conditions so that aconsiderable fraction of the carriers are captured by the compositionalinhomogeneous regions for radiative recombination. In an embodiment, acharacteristic size of the compositional inhomogeneous regions issmaller than 1/N_(reg) ^(0.5), where N_(reg) is the average density ofthe compositional inhomogeneous regions per unit area.

The area and the density of the compositional inhomogeneous regions 42also can affect a reliability and/or performance of a device. Forexample, radiation can lead to radiation-enhanced dislocation glide(REDG). REDG is characterized by a reduction of activation energy forglide velocity. The REDG shares features common with similar effects inpoint defects known as the recombination-enhanced defect reaction(REDR). To improve reliability of a device, the radiation andrecombination process can be configured to occur away from threadingdislocation cores in order to reduce radiation-enhanced dislocationglide. For example, for a case of compositional inhomogeneous regions 42having a small characteristic lateral area (e.g., as defined herein) andlow density (e.g., much smaller than the dislocation density), radiationemitted in those regions may be spatially isolated from the threadingdislocation cores 44 as long as the compositional inhomogeneous regions42 are located between the threading dislocation core regions. This canresult in improved reliability of the device.

Turning now to FIGS. 6A and 6B, in an embodiment, the plurality ofcompositional inhomogeneous regions 42 can be located along a planelocated proximate to (e.g., at or within a few atomic layers of) aninterface 46 between a quantum well 38 and a barrier 40. In thisembodiment, the compositional inhomogeneous regions 42 can be configuredto capture carriers for radiative recombination. To this extent, theregions 42 can have a depth of at least thermal energy. In a morespecific embodiment, the compositional inhomogeneous regions 42 have anenergy depth of at least one optical phonon energy.

In an embodiment, the semiconductor layers of the device 10 (FIG. 2) cancomprise an Al_(x)B_(y)In_(z)Ga_(1-x-y-z)N alloy with 0≦x≦1, 0≦y≦1,0≦z≦1, 0≦1-x-y-z≦1. Semiconductor heterostructures formed ofAl_(x)B_(y)In_(z)Ga_(1-x-y-z)N alloys contain polarization fields due tospontaneous polarization and piezo-polarization. This results in thetilting and bending of the band diagram, as shown, for example, in FIG.7A. In a particular embodiment, the active region 18 can include aheterostructure (e.g., a multiple quantum well structure) of barriersand quantum wells. An energy on one side of a conductive band for thequantum wells can be higher than the energy on the other side (e.g., atilted conductive band). Similarly, the valence band is called a“tilted” valence band. This tilting of the conductive band results inelectron localization in the region with lower energy, i.e., the lowside of the band diagram. In a more specific embodiment, thecompositional inhomogeneous regions 42 can be located along a plane 48located proximate to the low side of the conductive band of a quantumwell 38. In another embodiment, compositional inhomogeneous regions 42with greater depth can be located along a plane proximate to the highside of the conductive band of the quantum well 38, e.g., to promotetunneling of the electron wave function from the low side to the highside. This can result in a better spread of the electron wave function,improved overlap with holes, and/or the like.

In an embodiment shown in FIG. 8A, a semiconductor layer can includeboth large and small scale compositional inhomogeneous regions (whichcan be achieved, for example, by varying conditions of the epitaxialgrowth). The large scale compositional inhomogeneous regions 42A canhave large lateral areas of inhomogeneous regions. For example, thelateral area for the large scale compositional inhomogeneous regions 42Acan be configured to be larger than the square of the characteristicdistance between the threading dislocations 44 (which can be achieved,for example, by varying conditions of the epitaxial growth). An energydepth of the large scale compositional inhomogeneous regions 42A can beon the order of one thermal energy or more. The large scalecompositional inhomogeneous regions 42A can allow for an efficientcapture of the carriers, relative localization of the carriers withinthe large energy valleys (e.g., 42A in the band gap map of FIG. 8B),and/or the like. Subsequent localization of carriers can be due tocapture at the small scale compositional inhomogeneous regions 42B. Thesmall scale compositional inhomogeneous regions 42B also can have adepth on the order of one thermal energy or more. The small scalecompositional inhomogeneous regions 42B have a lateral area that issmaller than the square of the characteristic distance between thethreading dislocations 44. The small scale compositional inhomogeneousregions 42B allow for capturing the carriers before the carriers arecaptured by the threading dislocations 44.

In another embodiment shown in FIGS. 9A and 9B, a distribution of thecompositional inhomogeneous regions 42 can be graded. For example, adepth of the energy level across a quantum well 38 can increase ordecrease from an area proximate to a first barrier 40 to an areaproximate to a second barrier (not shown). In FIG. 9A, the distributionof the compositional inhomogeneous regions 42 is graded such that theenergy depth decreases towards the first barrier 40. In FIG. 9B, thedistribution of the compositional inhomogeneous regions 42 is gradedsuch that the energy depth increases toward the first barrier 40.

For quantum wells with tilted conduction bands, such as the quantumwells 38 shown in FIG. 7A, distributed grading of the compositionalinhomogeneous regions 42 can promote tunneling of carriers to theregions having higher band gaps. For example, in FIG. 10A, a quantumwell 38 is shown including a first localization region 50 and a secondlocalization region 52. The first localization region 50 can have adeeper energy band gap level than the second localization region 52,e.g., for tunneling to occur between the regions 50, 52. This tunnelingeffect can improve an overlap of the electron/hole wave function andpromote higher recombination rates. The black curved line included atthe bottom of the quantum well 38 in FIG. 10B corresponds to an electronwave function, which has a concentration in the corresponding portion ofthe quantum well 38.

Turning now to FIG. 11, a graded distribution of compositionalinhomogeneous regions can be combined or enhanced with compositionalgrading of a quantum well 38. For example, the quantum well 38 can havegraded composition that results in band bending at the first and/orsecond side of the quantum well 38. When the first side of the quantumwell 38 is a high side and contains deeper compositional inhomogeneousregions than the second side, the tunneling of carriers can be promotedby further reducing the energy of the conducting band at the first sidethrough compositional grading. Illustrative distributions of electron(top) and hole (bottom) wave functions are shown by the curved lines.

In an embodiment, multiple semiconductor layers in a device 10 (FIG. 2)can include compositional inhomogeneous regions. For example, turning toFIGS. 12A-12C, layers that include compositional inhomogeneous regionscan, in addition to improving carrier localization for radiativerecombination, affect the stress and strain in the device 10.Controlling the stress and strain within the device 10 can control thepropagation of threading dislocations throughout the layers of thedevice 10. For example, with semiconductor layers including group IIImaterials, if the compositional difference between the layers is atleast five percent in at least one molar fraction (e.g., x, y, z), thelayers with the compositional inhomogeneous regions can enhance orreduce compressive and tensile stresses in the semiconductor layers.

Controlling the stress and strain within the device 10 also can affectthe three dimensional growth of layers epitaxially grown above thelayers with compositional inhomogeneous regions. For example,compressive strain promotes three dimensional island formation, whiletensile strain promotes layer-by-layer two dimensional crystalformation. The type and magnitude of the strain can be used to controlthe compositional inhomogeneous regions. For example, adjacent layerswith compositional inhomogeneous regions that differ by at least a fewpercent in average band gap fluctuation amplitude, density, lateralsize, and/or the like, can be grown. FIG. 12C shows a layer 113C, whichincludes compositional inhomogeneous regions that differ from thecompositional inhomogeneous regions in layer 114C.

Although the embodiments shown in the figures include compositionalinhomogeneous regions in the quantum well, it is understood that thecompositional inhomogeneous regions can be included in any layer. Forexample, the barrier 40 (FIG. 3A) can include a plurality ofcompositional inhomogeneous regions, e.g., for stress/strain control.The plurality of compositional inhomogeneous regions in the barrier 40can control the stress/strain without altering the average band gapcharacteristic of the barrier 40. For example, in FIG. 12B, the layer114B includes a plurality of compositional inhomogeneous regions and isadjacent to a layer 113B grown at a high V/III ratio. The layer 112Bincludes a plurality of compositional inhomogeneous regions and isadjacent to the layer 111B that is grown at a low V/III ratio. In a moreparticular embodiment, FIG. 12B can be a multi-layer barrier comprisinga AlGaN/AlGaN superlattice to control the compressive strain in thequantum wells and also affect the compositional inhomogeneous regions inthe quantum wells, which can be misplaced relative to each other indifferent superlattice layers.

In another embodiment, a superlattice of semiconductor layers caninclude layers with relatively uniform composition alternating withlayers with compositional inhomogeneous regions. In FIG. 12A, layers111A and 113A have relatively uniform composition, while layers 112A and114A include compositional inhomogeneous regions. Relatively uniformcomposition includes compositional variation of less than three thermalenergies across the layer. In another embodiment, a layer in thesuperlattice of semiconductor layers can vary from an adjacent layer bymore than five percent in band gap amplitude, density, and/or the likefor the compositional inhomogeneous regions. Regardless, at least twolayers in the superlattice of semiconductor layers can be substantiallyequal (e.g., within a few (e.g., three) percent) in at least one of: adistribution grading, a band gap magnitude, a density, and/or the likeof the plurality of compositional inhomogeneous regions.

In another embodiment, variations in band gap can be achieved bylocalized doping. Turning to FIG. 13A, localized p-doping at a region56, which can be located anywhere within the semiconductor layer (e.g.,a quantum well), induces localized energy maxima for electrons 58 andlocalized energy minimum for holes 60. In an embodiment, the localizedp-doping inhomogeneities include a characteristic size less than theelectron Bohr radius. With the p-doping inhomogeneities, the electrons58 can tunnel through the energy maxima and recombine with holes 60localized at the hole energy minima. In a more specific embodiment, thep-doping can be located within the valleys of the compositionalinhomogeneous regions. For example, in FIG. 13B, the large scalecompositional inhomogeneous region 42A can contain the localized dopinginhomogeneities 56 within the valley of the small scale compositionalinhomogeneous region 42B. The valley of the small scale compositionalinhomogeneous region 42B can be defined as the region with theconduction energy value that is less than the average conduction bandenergy level. The p-doping inhomogeneities 56 can promote furthercarrier localization and carrier recombination. The localized p-dopingcan be an impurity, such as silicon, magnesium, beryllium, germanium,carbon, and/or the like.

Control over energy depth, distribution grading, lateral area size,and/or the like, of the compositional inhomogeneous regions can beachieved by controlling the epitaxial growth parameters during metalorganic chemical vapor deposition (MOCVD) growth. Alloy fluctuations canbe induced by fundamental difference in the mobility of the particularmetal (Al, Ga, In, etc.) adatoms on the surface at particular growthconditions. Therefore, compositional inhomogeneous regions can beregulated by controlling parameters which influence the mobility of theadatoms, such as growth temperature, V/III ratio, growth rate, andlayer-strain. Growth temperature in the range of 600-1300° C., V/IIIratio in the range of 10-50000, growth rate in the range of 1-200nm/min, and/or the like, can be used to create compositionalinhomogeneous regions. For example, in a specific embodiment,compositional inhomogeneous regions in Al_(0.5)Ga_(0.5)N can be enhancedby reducing growth temperature (e.g., less than 1200° C.) and increasingV/III ratio (e.g., greater than one hundred) at faster growth rates(e.g., greater than five nanometers/minute). Regardless, thesemiconductor layer including the compositional inhomogeneous regionscan be epitaxially grown partially or completely pseudomorphically(e.g., with the same lattice constant as the substrate) over anotherlayer or substrate. Partially pseudomorphic is defined as epitaxialgrowth with less than 95% degree of relaxation.

Turning now to FIGS. 14A-14C, illustrative topographic imagescorresponding to sample AlGaN layers with increasing Al molar fractionsare shown. The surface morphology image indicates the presence ofdefects and inhomogeneous regions in the material. In FIGS. 14A and 14B,the molar fraction of Al is increased from 0.3 to 0.42. FIG. 14B clearlyillustrates that the surface of the material has more pronouncedfeatures for the higher Al molar fraction. In FIG. 14C, the molarfraction of Al is increased to 0.5 and the defects and inhomogeneousregions are readily apparent. In a specific embodiment of the device,the active region can contain an AlN molar fraction of betweenapproximately twenty and approximately eighty percent.

Analysis of compositional inhomogeneous regions in a semiconductor layercan be performed using scanning near field optical microscopy (SNOM),which provides sub-wavelength spatial resolution. Electroluminescenceand photoluminescence (PL) SNOM studies of c-plane AlGaN quantum wells(QWs) have identified carrier localization and non-radiativerecombination centers. Furthermore, these studies reveal potentialbarriers around the extended defects. Near-field maps of the PL peakintensity, and peak energy, are presented for Al_(0.3)Ga_(0.7)N,Al_(0.42)Ga_(0.58)N, and Al_(0.5)Ga_(0.5)N layers in FIGS. 15A, 15C, and15C, respectively. Comparing the intensity of emission with peak energymap illustrates that the alloys containing low-to-modest molar ratios ofaluminum, such as a molar fraction of 0.3 or 0.42, have domain-likeareas emitting at red shifted wavelengths. As the aluminum content isincreased, domain-like structures give way to smaller compositionalinhomogeneous regions distributed uniformly throughout the structure.These compositional inhomogeneous regions have a red shifted emission.

Additionally, in FIGS. 15A and 15B, a correlation between the intensitypeak and the energy peak is shown. In particular, the red shiftedregions radiate at somewhat smaller intensity than blue shiftedcounterparts. This is consistent with growth models, where during theinitial growth stage, adjacent small islands, from which the growthstarts, coalesce into larger grains. As the islands enlarge, Ga adatoms,having a larger lateral mobility than Al adatoms, reach the islandboundaries more rapidly. Therefore, it is expected that the Gaconcentration in the coalescence be higher than in the center of theislands. At the same time, the coalescent boundaries contain largenumbers of defects. It is reasonable to expect lower emission intensityin these areas.

Turning now to FIGS. 16A-16C, illustrative maps corresponding to sampleAlGaN layers with increasing Al molar fractions are shown. The band gapillustrates how increasing the Al molar fraction increases the amplitudeof the band gap for the small scale compositional inhomogeneous regionsto be similar to the amplitude of the large scale compositionalinhomogeneous regions.

FIG. 17 shows a portion of a layer according to an embodiment of theinvention. The layer can include a plurality of domains 60 and thecompositional inhomogeneous regions can be grown to be away from thedomain boundaries 62, where a large concentration of threadingdislocations and defects are located. The semiconductor layer can begrown using Migration Enhanced Metalorganic Chemical Vapor Deposition(MEMOCVD), which has a higher growth rate than Molecular Beam Epitaxy(MBE). The exact growth rate of the MEMOCVD can be selected to controlthe diffusion length, di, of the Ga atoms, such that the diffusionlength is smaller than the average length, L, between threadingdislocations.

The average length, L, between threading dislocation cores is determinedby the density of the threading dislocations in the semiconductor layer.In an embodiment, a method of growth takes advantage of an approachdisclosed in U.S. Patent Application Publication No. 2014/0110754, whichis hereby incorporated by reference. The methods of growth discloses theart of epitaxial growth of semiconductor layers with low dislocationdensity due to growth of alternating compressive and tensile layers.FIGS. 18A and 18B show possible embodiments of the method, where in FIG.18A, the buffer layer is grown on a substrate with compressive layerfollowing the buffer layer alternating with tensile layer for severalperiods of epitaxial growth. FIG. 18B shows another embodiment of themethod in which the tensile layer is grown on the buffer layer withsubsequent compressive layer grown above the tensile layer. It isunderstood that the buffer layer is optional and may not be needed forsome embodiments.

The advantages of this method are shown in FIGS. 19A and 19B, whichillustrate the bright field optical microscope image of a layer grownwithout any strain modulation (FIG. 19A) and a layer with strainmodulation (FIG. 19B). As clearly seen from the figures, the number ofcracks (e.g., threading dislocations) is significantly reduced using thepresent methods. Other methods of analyzing dislocation density includeanalysis of a rocking curve full width at half maximum (FWHM) shown inFIG. 20. Reduction in AlN (102) XRD rocking curve FWHM, shown in FIG.20, as a function of layer thickness indicates reduced edge dislocationsdensity.

U.S. Patent Application Publication No. 2014/0110754 also provides thatdislocation reduction may be obtained by reducing build up stress insemiconductor layers by patterning the substrate, the buffer layer,and/or one or more of the semiconductor layers. FIGS. 21A and 21B showthat patterning of a substrate and/or intermediate semiconductor layerscan be employed to produce compressed and tensile layers having a commonboundary not only in vertical direction of growth, but also, in laterallayer direction. Possible patterns comprise stripes, rectangular windows66, and/or the like. Also, the relative position of patterning elementsbetween sets of layers may be varied. For example, in one embodiment,the position of patterning elements on one layer may form acheckerboard-like formation with the patterning elements on anotherlayer (FIG. 21A). Alternatively, in an embodiment, the position ofpatterning element on one layer may be the same lateral location as thepatterning elements on another layer (FIG. 21B).

In an embodiment, a contact can be formed for a semiconductor layerincluding a plurality of compositional inhomogeneous regions. Forexample, FIG. 22 shows a contact 80 for a semiconductor layer 15including a plurality of compositional inhomogeneous regions (not shown)according to an embodiment. The semiconductor layer 15 is locatedbetween the substrate 12 and a buffer layer 14 and can include anyembodiment of compositional inhomogeneous regions discussed herein. Aplurality of metallic protrusions 82 extend from the contact 80 throughthe buffer layer 14 in order to contact the semiconductor layer 15.

The plurality of metallic protrusions 82 can be formed using anysolution. For example, the plurality of metallic protrusions 82 can beformed by etching the buffer layer 14 and at least a portion of thesemiconductor layer 15 prior to the deposition of the metallicprotrusions 82 and the contact 80. In another example, selectiveovergrowth can be used when growing the semiconductor layer 15 and thebuffer layer 14 prior to the deposition of the metallic protrusions 82and the contact 80. The metallic protrusions 82 and the contact 80 canbe deposited through evaporation or a sputtering technique followed by asubsequent annealing. Alternatively, instead of the etching or selectiveovergrowth technique, the buffer layer 14 and the semiconductor layer 15can be grown to contain a plurality of voids or pores for the pluralityof metallic protrusions 82. A porous semiconductor layer including aplurality of voids can be formed by utilizing 3-dimensional (3D) growthtechniques for semiconductor layers. The buffer layer 14 can comprise ahigh adhesion to the metallic contact 80 and the semiconductor layer 15can be a highly conductive layer. In an embodiment, the buffer layer 14can have a higher aluminum nitride molar fraction than the averagealuminum nitride molar fraction of the semiconductor layer 15. In anembodiment, the semiconductor layer 15 can be a thin layer with athickness of approximately 10 nanometers (nm) to approximately 300 nm.In an embodiment, the thickness of the semiconductor layer 15 iscomparable to the length of the plurality of metallic protrusions 82.The buffer layer 14 can be partially transparent to radiation (e.g., atleast 30% of the radiation is transmitted through the buffer layer 14)when the radiation is normal to the surface of the layer 14.

Turning now to FIGS. 23A and 23B, a contact for a semiconductor layerincluding a plurality of compositional inhomogeneous regions accordingto embodiments is shown. In FIG. 23A, the semiconductor layer 15 caninclude a set of alternating sublayers (e.g., a first sublayer 17A and asecond sub layer 17B). Although only four sublayers are shown, it isunderstood that any number of sublayers can be included. Further,although only two types of sublayers are shown, it is understood thatany number of types of sublayers can be included. In an embodiment, anaverage bandgap for the first sublayer 17A is different than the averagebandgap of the second sublayer 17B. For example, the first sublayers 17Acan comprise quantum well structures including a plurality ofcompositional inhomogeneous regions 42 (FIG. 3A), while the secondsublayers 17B can comprise barrier structures (having a wider bandgapthan quantum wells) including a plurality of compositional inhomogeneousregions 42. In another embodiment, the variance of the bandgap for thefirst sublayer 17A can be different than the variance of the bandgap forthe second sublayer 17B. The variance refers to the degree of variationof the bandgap within a layer. For example, a bandgap variation of 100meV refers to a layer with fluctuations in the bandgap on the order of100 meV.

In an embodiment, the inhomogeneous regions in the second sublayer 17Bcan form a sufficiently dense structure to allow percolation. Forexample, the structure of the second sublayer 17B (e.g., barrier) cancomprise an Al_(x)Ga_(1-x)N layer with a varying molar fraction x. Thevariation in the molar fraction x allows for variation in the bandgapenergies within the second sublayer 17B. That is, some regions withinthe second sublayer 17B can have higher bandgap energies, while otherregions can have lower bandgap energies. The regions in the secondsublayer 17B with the lower bandgap energies have a sufficient densityso that there is an overlap of such regions or close proximity of suchregions, which leads to an interconnected (or percolated) low bandgapstructure. Such a structure can promote conductivity of the barrierlayer 14. It is understood that the embodiment shown in FIG. 22 can alsosupport a percolated low bandgap structure in the semiconductor layer15. It is also understood that the first sublayer 17A can containstructures that allow percolation.

A 2-dimensional (2D) gas is formed at the interface of a quantumwell/barrier (e.g., the interface between the first sublayer 17A and thesecond sublayer 17B). The formation of a 2D gas is particularlyimportant for semiconductors which have a large degree of polarization,such as group III nitrides (e.g., AlGaN, and/or the like). The regionsforming the 2D gas are contacted by the plurality of metallicprotrusions 82. Due to the inhomogeneous regions at the interface of thequantum well and barrier (e.g., the first sublayer 17A and the secondsublayer 17B), the 2D gas can have a diffusive profile and can partiallypenetrate through the second sublayer 17B (e.g., barriers) in theregions with low bandgap energies. The conductivity of the contact 80can be improved by the plurality of metallic protrusions 82 penetratingthe second sublayer 17B (e.g., barriers). In order to have a sufficientconductivity, each of the plurality of metal protrusions 82 are locatedat a distance away from each other that does not exceed the currentspreading length in the semiconductor layer 15.

Turning to FIG. 23B, a contact for a semiconductor layer including aplurality of inhomogeneous regions according to an embodiment is shown.In this embodiment, the semiconductor layer 15 can include a pluralityof domains 84A, 84B that have compositional inhomogeneous regions. Theplurality of domains 84A, 84B can form any structure, including largerdomains that have several protrusions embedded into them, or smallerdomains. The plurality of domains 84A, 84B can have a lateral dimensionsranging from 1 nanometer (nm) to several microns. A plurality ofmetallic protrusions 82 from a contact 80 extend into each of thedomains 84A, 84B. In an embodiment, each domain 84A, 84B can include alower aluminum nitride molar fraction as compared to the average molarfraction of aluminum within the semiconductor layer 15 and result inregions having higher conductivity. The domains 84A, 84B can be formedusing any solution, such as, for example, by depositing a material intoan etched valley in the semiconductor layer 15.

A plurality of compositional inhomogeneous regions in a semiconductorlayer including compositional inhomogeneous regions can increase thediffusion of a metallic contact through the semiconductor layer duringthe process of annealing, depending on the semiconductor'scharacteristics. For example, in FIG. 24, an illustrative contact 180 toa semiconductor layer 15 including a plurality of compositionalinhomogeneous regions according to an embodiment is shown. Thesemiconductor layer 15 can include a graded composition with anamplitude that varies throughout the layer. In an embodiment, theannealing of the contact 180 can depend on the composition of thesemiconductor layer 15. For example, the annealing of the contact 180can depend on the lattice quality of the semiconductor layer 15. In anembodiment, the lattice containing a large number of dislocations andinhomogeneous regions can promote a deep penetration of the contact 180into the semiconductor layer 15 (via metallic protrusion 182). This canincrease the diffusion area of the metal of the contact 180 into thesemiconductor layer 15. Therefore, a semiconductor layer 15 including agraded composition allows for additional control during the process ofcontact annealing. For example, a layer with compositional inhomogeneousregions can be grown using three dimensional growth and allow forimproved diffusion of the metallic contact 180 into the semiconductorlayer 15, which provides improved metal penetration into thesemiconductor layer 15, and as a result, improved ohmic properties ofthe contact 180. Additionally, the semiconductor layer 15 can provideimproved alloying (e.g., mixing) of the metallic regions within thesemiconductor layer 15. In an embodiment, the degree of mixing can becontrolled by the degree of compositional inhomogeneous regions withinthe semiconductor layer 15. Each metallic protrusion 182 can becharacterized by a diffusion distance DA, which is generally indicativeof the distance that the metallic elements associated with the metallicprotrusion 182 diffuses within the semiconductor layer 15. Eachsemiconductor layer has a mobility and overall quality that determinesthe spread or diffusion of carriers and is characterized by a carrierdiffusion length D_(l). In an embodiment, the distance between adjacentprotruding metallic protrusions 182 is selected to be comparable to acarrier diffusion length D_(l). Another length scale present in thedesign is a diffusion distance D_(A). The density of metallicprotrusions 182 can be estimated based on these two length scales asfollows: N=1/(π(D_(A)+D_(l))²), where N is the number of metallicprotrusions per unit area. In an embodiment, the length scale Di can besubstituted by the current spreading length, which can be approximatedas: D_(L)=√{square root over (2D_(A)(rb)/a tan(2rb/D_(A)))}, where b isthe semiconductor layer thickness, and r=ρ_(⊥)ρ_(∥), where ρ_(∥) is aresistivity along the semiconductor layer direction and ρ_(⊥) is aresistivity in the layer normal direction.

In an embodiment, etching the surfaces of a semiconductor layer canimprove annealing of a contact to a semiconductor layer. Turning now toFIGS. 25A-25C, illustrative etched surfaces of a semiconductor layeraccording to embodiments are shown. FIG. 25C shows that a buffer layer14 and/or a semiconductor layer 15 including a plurality ofcompositional inhomogeneous regions can be etched. As shown in FIG. 25A,in an embodiment, wet etching can be used. The parameters of the wetetching are selected to produce porous morphology on the scale ofapproximately 20 nanometers (nm). In another embodiment, as shown inFIG. 25B, larger pores can have a characteristic scale of approximately40 nm to approximately 60 nm. In general, wet etching is selected toproduce the optimal metallic contact after annealing. In an embodiment,optimal parameters for etching are selected in order to produce optimaloptical properties of the metal-semiconductor interface (e.g., theinterface between the contact 80 and the semiconductor layer 15 (FIG.22)) for optical scattering. To select the optimal parameters, such ascomposition of the bath, temperature, presence of light, duration ofetching, presence of catalyst, and/or the like, for etching, thescattering and contact characteristics can be evaluated and interpolatedfor several etching processes. In an embodiment, wet etching is selectedto produce the porous morphology that is at least on the order of thetarget wavelength of the light emitting or light absorbing device.

In an embodiment, wet etching the semiconductor layer 15 results in aporous morphology that is correlated to the length scales of thecompositional inhomogeneous regions within the semiconductor layer 15.During the wet etching process, regions with a higher aluminum molarfraction are etched more than domains with higher gallium nitride molarfractions. In an embodiment, the wet etching can be accompanied byelectro-chemical etching, photo-chemical etching, or a combination. In afurther embodiment, a photo-chemical etching can further control thelength scales of the pores generated throughout the etching process.

In another embodiment, dry etching can also be used either independentlyof wet etching, or after the wet etching process. Dry etching canproduce variations on the surface structure (of the barrier layer 14surface and/or the semiconductor layer 15 surface) on the scale ofapproximately 0.5 micrometers to approximately 50 micrometers. In anembodiment, masking the area prior to etching can result in theformation of user determined patterns (e.g., a periodic structure). Forexample, such a structure can comprise a photonic crystal.

In an embodiment, non-uniform etching can be applied to a semiconductorlayer including compositional inhomogeneous regions. Turning now toFIGS. 26A and 26B, illustrative etched surfaces of a semiconductor layerwith non-uniform etching according to embodiments is shown. FIG. 26Ashows that the etching can be different at the sides (e.g., along thewidth, along the perimeter, and/or the like) of the semiconductor layer15 and the middle of the semiconductor layer 15. For example, a size anda depth of an etching domain 86A located at the side of thesemiconductor layer 15 can be different than a size and a depth of anetching domain 86B located in the middle of the semiconductor layer 15.In an embodiment, as shown in FIG. 26B, the etching can vary both in anx direction and a y direction. A dark region 88A corresponds to anetching that is different than an etching in a lighter region 88B. Thedifference in the etching process in each region/domain can includemasking, several etching steps, electrochemical and photochemicaletching, and/or the like. Additionally, the difference in the etchingprocess can include initially preparing the semiconductor layer 15 witha variation in the plurality of compositional inhomogeneous regions. Forexample, the variation in the plurality compositional inhomogeneousregions can be formed by patterning and masking or can be a byproduct ofthe Metalorganic Chemical Vapor Deposition (MOCVD) growth process. Dueto the presence of compositional inhomogeneous regions, the areas havinghigher aluminum nitride content are etched at a higher rate that areashaving a higher gallium nitride content.

In one embodiment, the invention provides a method of designing and/orfabricating a circuit that includes one or more of the devices designedand fabricated as described herein (e.g., including one or more devicesfabricated using a semiconductor structure described herein). To thisextent, FIG. 27 shows an illustrative flow diagram for fabricating acircuit 1026 according to an embodiment. Initially, a user can utilize adevice design system 1010 to generate a device design 1012 for asemiconductor device as described herein. The device design 1012 cancomprise program code, which can be used by a device fabrication system1014 to generate a set of physical devices 1016 according to thefeatures defined by the device design 1012. Similarly, the device design1012 can be provided to a circuit design system 1020 (e.g., as anavailable component for use in circuits), which a user can utilize togenerate a circuit design 1022 (e.g., by connecting one or more inputsand outputs to various devices included in a circuit). The circuitdesign 1022 can comprise program code that includes a device designed asdescribed herein. In any event, the circuit design 1022 and/or one ormore physical devices 1016 can be provided to a circuit fabricationsystem 1024, which can generate a physical circuit 1026 according to thecircuit design 1022. The physical circuit 1026 can include one or moredevices 1016 designed as described herein.

In another embodiment, the invention provides a device design system1010 for designing and/or a device fabrication system 1014 forfabricating a semiconductor device 1016 as described herein. In thiscase, the system 1010, 1014 can comprise a general purpose computingdevice, which is programmed to implement a method of designing and/orfabricating the semiconductor device 1016 as described herein.Similarly, an embodiment of the invention provides a circuit designsystem 1020 for designing and/or a circuit fabrication system 1024 forfabricating a circuit 1026 that includes at least one device 1016designed and/or fabricated as described herein. In this case, the system1020, 1024 can comprise a general purpose computing device, which isprogrammed to implement a method of designing and/or fabricating thecircuit 1026 including at least one semiconductor device 1016 asdescribed herein.

In still another embodiment, the invention provides a computer programfixed in at least one computer-readable medium, which when executed,enables a computer system to implement a method of designing and/orfabricating a semiconductor device as described herein. For example, thecomputer program can enable the device design system 1010 to generatethe device design 1012 as described herein. To this extent, thecomputer-readable medium includes program code, which implements some orall of a process described herein when executed by the computer system.It is understood that the term “computer-readable medium” comprises oneor more of any type of tangible medium of expression, now known or laterdeveloped, from which a stored copy of the program code can beperceived, reproduced, or otherwise communicated by a computing device.

In another embodiment, the invention provides a method of providing acopy of program code, which implements some or all of a processdescribed herein when executed by a computer system. In this case, acomputer system can process a copy of the program code to generate andtransmit, for reception at a second, distinct location, a set of datasignals that has one or more of its characteristics set and/or changedin such a manner as to encode a copy of the program code in the set ofdata signals. Similarly, an embodiment of the invention provides amethod of acquiring a copy of program code that implements some or allof a process described herein, which includes a computer systemreceiving the set of data signals described herein, and translating theset of data signals into a copy of the computer program fixed in atleast one computer-readable medium. In either case, the set of datasignals can be transmitted/received using any type of communicationslink.

In still another embodiment, the invention provides a method ofgenerating a device design system 1010 for designing and/or a devicefabrication system 1014 for fabricating a semiconductor device asdescribed herein. In this case, a computer system can be obtained (e.g.,created, maintained, made available, etc.) and one or more componentsfor performing a process described herein can be obtained (e.g.,created, purchased, used, modified, etc.) and deployed to the computersystem. To this extent, the deployment can comprise one or more of: (1)installing program code on a computing device; (2) adding one or morecomputing and/or I/O devices to the computer system; (3) incorporatingand/or modifying the computer system to enable it to perform a processdescribed herein; and/or the like.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to anindividual in the art are included within the scope of the invention asdefined by the accompanying claims.

What is claimed is:
 1. A device comprising: a semiconductor layercomprising a plurality of compositional inhomogeneous regions, wherein adifference between an average band gap for the plurality ofcompositional inhomogeneous regions and an average band gap for aremaining portion of the semiconductor layer is at least 26 meV, andwherein the semiconductor layer includes a first side and a second side,and wherein a distribution of the plurality of compositionalinhomogeneous regions is graded, such that the first side has an averageband gap higher than an average band gap of the second side.
 2. Thedevice of claim 1, wherein a characteristic distance between eachdislocation is greater than a smallest characteristic size for acompositional inhomogeneous region.
 3. The device of claim 1, wherein anaverage distance between the compositional inhomogeneous regions is lessthan an ambipolar diffusion length.
 4. The device of claim 1, whereinthe average band gap for the plurality of compositional inhomogeneousregions comprises an energy of at least one optical phonon.
 5. Thedevice of claim 1, wherein a characteristic size of the plurality ofcompositional inhomogeneous regions is smaller than an inverse of adislocation density for the semiconductor layer.
 6. The device of claim1, wherein the semiconductor layer is one of: a barrier or a quantumwell in a multiple quantum well structure of an active region of thedevice, and wherein the plurality of compositional inhomogeneous regionsare located in proximity to an interface between the barrier and thequantum well.
 7. The device of claim 1, further comprising a metalliccontact for the semiconductor layer, wherein the metallic contactincludes a plurality of metallic protrusions extending into thesemiconductor layer.
 8. The device of claim 7, wherein the semiconductorlayer includes a set of sublayers, each sublayer including a pluralityof compositional inhomogeneous regions, and wherein an average bandgapof a first sublayer is different from an average bandgap of a secondsublayer.
 9. The device of claim 8, wherein a density of the pluralityof compositional inhomogeneous regions within the first sublayer forms apercolated network.
 10. A device comprising: a semiconductor structureincluding an active region, wherein the active region comprises amultiple quantum well structure including: a plurality of barriersalternating with a plurality of quantum wells, wherein at least one of:a barrier in the plurality of barriers or a quantum well in theplurality of quantum wells includes a plurality of compositionalinhomogeneous regions, wherein a difference between an average band gapfor the plurality of compositional inhomogeneous regions and an averageband gap for a remaining portion of the semiconductor layer is at least26 meV, and wherein a characteristic size of the plurality ofcompositional inhomogeneous regions is smaller than an inverse of adislocation density for the semiconductor layer, wherein the averageband gap for the plurality of compositional inhomogeneous regionscomprises an energy of at least one optical phonon.
 11. The device ofclaim 10, wherein the plurality of compositional inhomogeneous regionsare located in proximity to an interface between a quantum well and abarrier.
 12. The device of claim 10, wherein an average distance betweenthe compositional inhomogeneous regions is less than an ambipolardiffusion length.
 13. The device of claim 10, wherein the at least onebarrier or the at least one quantum well includes a first side and asecond side, and wherein a distribution of the plurality ofcompositional inhomogeneous regions is graded, such that the first sidehas an average band gap higher than an average band gap of the secondside.
 14. The device of claim 10, wherein at least one of: adistribution grading, magnitude, or density of the plurality ofcompositional inhomogeneous regions is substantially equal for at leasttwo quantum wells or at least two barriers.
 15. The device of claim 10,wherein the plurality of compositional inhomogeneous regions in a firstlayer is at least 5% different from the plurality of compositionalinhomogeneous regions in a second layer adjacent to the first layer. 16.The device of claim 10, wherein each barrier in the plurality ofbarriers and each quantum well in the plurality of quantum wellsincludes a plurality of compositional inhomogeneous regions, and furthercomprising a metallic contact including a plurality of metallicprotrusions extending into each of the plurality of compositionalinhomogeneous regions.
 17. A method comprising: forming an active regionof a semiconductor structure, wherein the active region comprises alight emitting heterostructure, the forming including: forming aplurality of barriers alternating with a plurality of quantum wells,wherein forming at least one of: a barrier in the plurality of barriersor a quantum well in the plurality of quantum wells includes forming aplurality of compositional inhomogeneous regions, wherein an averageband gap for the plurality of compositional inhomogeneous regionsexceeds 26 meV, and a characteristic size for each compositionalinhomogeneous region is smaller than an inverse of a dislocationdensity, and wherein an average distance between the compositionalinhomogeneous regions is less than an ambipolar diffusion length. 18.The method of claim 17, wherein forming the plurality of compositionalinhomogeneous regions includes at least one of: adjusting a V/III ratio,adjusting a growth temperature, or adjusting a composition duringepitaxial growth of the at least one of: the barrier in the plurality ofbarriers or the quantum well in the plurality of quantum wells.
 19. Themethod of claim 17, wherein forming the plurality of compositionalinhomogeneous regions includes forming a characteristic size for eachcompositional inhomogeneous region that is smaller than an inverse of adislocation density.
 20. The method of claim 17, further comprising:etching the at least one of the barrier or the at least one quantum wellincluding the plurality of compositional inhomogeneous regions; andforming a metallic contact including a plurality of metallicprotrusions, wherein each of the plurality of metallic protrusionsextends into the plurality of barriers alternating with the plurality ofquantum wells to contact the plurality of inhomogeneous regions.